The production of oil and natural gas from an underground well (subterranean formation) can be stimulated by a technique called hydraulic fracturing, in which a viscous fluid composition (fracturing fluid) containing a suspended proppant (e.g., sand, bauxite) is introduced into an oil or gas well via a conduit, such as tubing or casing, at a flow rate and a pressure which create, reopen and/or extend a fracture into the oil- or gas-containing formation. The proppant is carried into the fracture by the fluid composition and prevents closure of the formation after pressure is released. Leak-off of the fluid composition into the formation is limited by the fluid viscosity of the composition. Fluid viscosity also permits suspension of the proppant in the composition during the fracturing operation. Cross-linking agents, such as borates, titanates or zirconates are usually incorporated into the composition to control viscosity.
Normally, less than one third of available oil is extracted from a well after it has been fractured before production rates decrease to a point at which recovery becomes uneconomical. Enhanced recovery of oil from such subterranean formations frequently involves attempting to displace the remaining crude oil with a driving fluid, e.g., gas, water, brine, steam, polymer solution, foam, or micellar solution. Ideally, such techniques (commonly called flooding techniques) provide a bank of oil of substantial depth being driven into a producing well; however, in practice this is frequently not the case. Oil-bearing strata are usually heterogeneous, some parts of them being more permeable than others. As a consequence, channeling frequently occurs, so that the driving fluid flows preferentially through zones depleted of oil (so-called “thief zones”) rather than through those parts of the strata which contain sufficient oil to make oil-recovery operations profitable.
Difficulties in oil recovery due to high permeability of zones may be corrected by injecting an aqueous solution of an organic polymer and a cross-linking agent into certain subterranean formations under conditions where the polymer will be cross-linked to produce a gel, thus reducing the permeability of such subterranean formations to driving fluid (gas, water, etc.). Polysaccharide- or partially hydrolyzed polyacrylamide-based fluids cross-linked with certain aluminum, titanium, zirconium and boron-based compounds are also used in these enhanced oil recovery applications.
Cross-linked fluids or gels, whether for fracturing a subterranean formation or for reducing permeability of a subterranean formation, are now being used in hotter, deeper wells under a variety of pH conditions, where rates of cross-linking with known cross-linking compositions may be unacceptable. Rather than developing new cross-linking agents for these new conditions, the oil well service companies may add delay agents that effectively delay the cross-linking of a particular metal cross-linking agent under these conditions.
A number of patents disclose the use of various delay agents in combination with particular cross-linking agents for which they are effective. These patents typically specify adding one or more ingredients to a cross-linking composition or specify particular operating conditions, such as a narrow range of pH. There are only a limited number of disclosed delay agents suitable for titanium and zirconium cross-linking agents. Thus, use of delay agents with titanium and zirconium cross-linking agents has limited flexibility for use by the oil well service companies to stimulate or enhance recovery of oil or gas from a well or other subterranean formation.
There is a need for a cross-linking composition which is effective for delaying the action of titanium and zirconium cross-linking agents in oil recovery applications over a range of conditions. There is a need to be able to control rate of cross-linking so as to provide flexibility to a cross-linking agent, so that a range of cross-linking rates and may be achieved under a range of pH conditions with a single cross-linking composition. The present invention meets these needs.